In a light emitting device such as a light emitting diode (LED) and a photocoupler, a composition for sealing a light emitting element is required not only to have a function of protecting the light emitting element from an external part but also to have satisfactory and stable adhesiveness to the light emitting element or a support substrate of polyphthalamide, ceramics, or the like supporting the light emitting element. The composition is also required to have high transparency so as not to decrease luminance of the light emitting element.
As such a sealing composition, for example, epoxy resin and the like have been conventionally used. However, in LEDs and the like, in accordance with the recent increase in luminance, a heat generation amount has been increasing and the wavelength of light has been shortened, and thus the use of the epoxy resin has been a cause of cracking and yellowing.
Therefore, as a sealing composition excellent in heat resistance and ultraviolet resistance, an organopolysiloxane component (silicone composition) has been used. In particular, an addition-reaction type silicone composition utilizing hydrosilylation has been widely used since it is excellent in productivity because of its quick curabililty by heating and it produces no byproduct when cured.
As the addition-reaction type silicone composition, for example, a patent document 1 describes a silicone composition comprising: a diorganopolysiloxane containing at least two alkenyl groups, each alkenyl group being bonded to a silicone atom per molecule; an organopolysiloxane which contains SiO4/2 units, Vi(R3)2SiO1/2 units, and R33SiO1/2 units (where Vi represents a vinyl group, R3 represents an unsubstituted or substituted monovalent hydrocarbon group containing no aliphatic unsaturated bond); an organohydrogenpolysiloxane containing, per molecule, at least two hydrogen atoms, each hydrogen atom being bonded to a silicon atom; and a platinum group metal catalyst.
However, the silicone composition described in the patent document 1 after cured has a higher coefficient of linear expansion than a support substrate and thus has a high residual stress after heated and cured, which has posed problems that it peels off on an interface with the support substrate and the deformation of the support substrate occurs (see, for example, Patent Reference 1).    Patent Reference 1: JP-A 2000-198930 (KOKAI)